How fast does time move? The answer depends on what you’re thinking.

Time is fixed. A second is a second, there’s no question about it. Or so people thought.

Then came Albert Einstein, with his theories of relativity. He showed that a second doesn’t have to be a second: it can also be a minute or a year, depending on how fast you’re travelling. You don’t usually notice, because the effects are so tiny. But hop onto a superfast spacecraft, take a ten-day holiday travelling near the speed of light, and you’ll find decades have gone by at home.

“Wow!” you’ll say. “Time really does fly when you’re having fun!”

Actually, Einstein himself said something similar. “When you are courting a nice girl an hour seems like a second,” he joked. “When you sit on a red-hot cinder a second seems like an hour. That’s relativity.”

But does time really go slower in unpleasant situations and speed up when you’re enjoying yourself? Looking back, it doesn’t seem so.

The break’s over, and you’re just returning from a fun holiday. “What?” you exclaim. “Is it time already?” It seems like your holiday-time’s just begun, but real-world time has already moved forward by ten days. No rockets required.

But here’s the paradox: when you look back on the holiday later, it feels like you’ve spend more than ten days there, not less.

How can something seem to last for more time as well as less? It’s because your mind has two different modes of looking at things: experiencing events directly, or looking back and remembering them later.

The patients were going through colonoscopy and lithotripsy: both painful processes that could take over an hour. And just how painful was it? That’s what psychologist Daniel Kahneman, along with doctor Donald Redelmeier, was aiming to find out.

Every sixty seconds, patients were asked to rate their pain levels on a scale from one to ten. At the end, they were asked to give a final rating, of how painful they thought the operation was overall.

Kahneman found that the final rating didn’t depend on the moment-by-moment pain readings. It was heavily influenced by only two things: the ‘peak’ moment of highest pain, and the level at the end of the procedure.

This ‘Peak-End’ rule was very powerful. A short mess-up at the end of the procedure could make people remember the whole operation as painful. After enduring long stretches of intense pain, other patients would feel better just because of a few minutes’ respite at the end. “That wasn’t so bad after all,” they’d say.

What is time? Physicists would say it’s the dimension along which things happen; the gap between cause and effect. Actually, the question has never been satisfactorily answered.

Time is causing physicists a lot of problems, because the two ways of describing the Universe — General Relativity and Quantum Mechanics — look at time in two very different ways. General Relativity thinks of it as a dimension of ‘space-time’, that can be warped and stretched according to certain rules. But Quantum Mechanics sees it in a more conventional way, with one second following another.

People haven’t yet found a way make the two work together, especially when looking at gravity in terms of space-time vibrations. As physicists Carlo Rovelli once said: What is a vibrating time?

Some theories say time is an emergent phenomenon, like a pointillist painting (it’s just a series of dots; speaking of ‘pictures’ makes sense only when you zoom out). Others propose that time is an illusion; they try describing the Universe without bringing time in at all.

In the living world, however, time is more like a tool. It’s a guide to make sense of what’s happened before, and to try estimating what will happen next.

You may not think of time as an invention, but a lot of its aspects are certainly made-up. Take the question “What’s the time?” That assumes every instant has a certain fixed rate, like “two thirty in the evening” or “ten o’clock AM”, which is the same for everybody. True, there are different timezones, but everyone within the timezone has the same clock, which is synchronised with UTC at a certain pre-decided offset.

We measure these differences very closely. GPS satellites even adjust their clocks to account for General Relativity: their time is a bit faster as they spin round the Earth.

But this ideas of synchronised clocks only came about in 1883, because of the British railway system. Before that, each town had its own local time, making railway timetables very confusing.

Stop reading for a moment. Get up and look in the mirror. (No, a selfie won’t do).

Can you see your face in the reflection? Good. Now focus on your right eye. You’ll see it staring back at you. Focus on your left eye, which will do the same. Keep switching back and forth between the eyes.

Now here’s the thing: between looking at your right eye and looking at your left, your eyeballs have to move from one eye to another. But you didn’t see them move, did you?

You can’t. Your brain edits it out.

This eyeball experiment was described in a paper by David Eagleman. He’s a neuroscientist who studies time — or, more specifically, how it is perceived by the human brain.

One of his famous studies is on near-death experiences.

f you haven’t had one yourself, you’ve probably read it in books: you’re falling off a cliff, or in the middle of a car-crash, or whatever it is, and you suddenly have a moment of clarity. Everything starts moving in slow-motion, and you become aware of every detail that’s going on.

Of course, time doesn’t really slow down — not from the perspective of most people, anyway. What happens is that your mind speeds up, so everything seems to slow down in comparison.

Eagleman’s question was: does your body really speed up, or does it only seem that way when you remember it later? To find out, he conducted an experiment with a stopwatch and a flashing screen. And, a near-death experience.

The Suspended Catch Air Device, or SCAD, has a complicated name — but the concept is simple. You get into a harness that is lifted up into the air. Then the ropes are released.

You plummet down a hundred-and-fifty feet, only to be caught at the last moment by a net at the bottom.

This somewhat unusual amusement-park ride is where David Eagleman is conducting his experiment. He has given you an assignment: a digital number display was strapped to your wrist, and you have to keep watching it as you fell. The digits are flashing just a bit too fast for the your brain to pick up — but if it speeds up even a little bit, in course of your ‘near-death’ experience, you’ll be able to see the numbers.

Eagleman times your descent. When it’s all over, he asked you to go through the experience in your head, with a stopwatch, to estimate the time it took.

The result? You, like most people, overestimate the time. You think your fall lasted nearly 37% longer than it actually did. But, you could never see any of the numbers on the display. That means, your mind never sped up as you were falling.

Instead, the slowing happened only later, when you played the memories back in your head.

Why does time seem different at different times? Why don’t you remember things exactly as they happened?

Well, that’s not the point of remembering.

Your brain doesn’t simply sum up your experiences. Instead of keeping track of every boring detail, your memory knows it’s the highlights that count. You can see this editing right in the eyeball experiment; you’ll notice it too in the ‘Peak-End’ rule when the brain tries turning your experiences into a story.

As doctor Atul Gawande once pointed out, you don’t remember your life as merely the sum of all its moments — that would be “mostly nothing much plus some sleep”.

As for time — well, the purpose of time is to make sense of the world. And when sense is a story, some experiences take up more space than others. They work like descriptions in a book, where an uneventful week can pass in a sentence, while a slow, detailed, momentous instant can go on for pages.

You don’t have to fall off a tower to experience the changing of time. You can even see it in the ‘oddball experiment’.

Look at the animals flashing on the screen. How long does each one stay?

Did the cat seem to stay for longer? Actually, all of them was displayed for exactly the same number of milliseconds. It seemed longer because you were paying more attention to the new creature. Your brain was taking more notes.

You can see this even in a clock-face: look at it quickly, and the second-hand will seem to stand still for a moment.

When you’re doing something engaging— or having an eventful holiday — your mind is so taken in by the new experience that it doesn’t notice the moments go by. But, when these moments are written down in the pages of your memory, they’re so full of details that they end up feeling thicker.

Imagine yourself back in the days before clocks. Time is more fluid; less exact. There’s no concept of lunch at 1:30 or getting up at 7. You would follow some sort of pattern, eating when the sun was up and sleeping after darkness, but it wouldn’t be so precise and second-bound and exact.

Days will be longer or shorter — depending on the seasons, yes, but also on your mental state and what you’ve been doing that day.

As Belle Beth Cooper, Content Crafter at Buffer, pointed out, you can even use the trick to make your days longer. Do something engaging, and you’ll end up having more time in the same amount of minutes.

You’ll realise something that, in this age of scheduled meetings and alarm-clocks, it’s easy to forget —

Time is what you think it is.

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